In recent years, various electronic devices such as liquid crystal display elements, organic EL (electroluminescence) elements, electronic paper, solar batteries, and thin film lithium ion batteries are made lighter and thinner. It is known that many of these devices are denatured and deteriorated by water vapor in the air.
Conventionally, in these devices, a glass substrate has been used as a support substrate. However, the use of a resin substrate instead of a glass substrate is now being considered, because the resin substrate is superior in various properties such as lightness, impact resistance, and flexibility. In general, resin substrates exhibit much higher permeability to gases such as water vapor than substrates formed of inorganic materials such as glass. Therefore, in the above-described application purposes, it is required that the gas barrier properties of the resin substrate are improved while maintaining the light permeability of the resin substrate.
It is noted that, electronic devices are required to achieve gas barrier properties extraordinarily higher than those of food packages. Gas barrier properties are represented by a water vapor transmission rate (referred to as “WVTR” hereinafter), for example. The value of WVTR required for conventional food packages is about 1 to 10 g·m−2·day−1. In contrast, it is considered that a WVTR of 1×10−3 g·m−2·day−1 or less is required for substrates to be used in thin film silicon solar batteries and compound thin film solar batteries, and a WVTR of 1×10−5 g·m−2·day−1 or less is required for substrates to be used in organic EL elements, for example. In order to address such very high requirements regarding the gas barrier properties, various kinds of methods in which a gas barrier layer is formed over a resin substrate have been proposed (see Patent Documents 1 and 2, for example). However, gas barrier properties of inorganic films formed by vacuum processes typified by these techniques still cannot satisfy the above-described requirements.
On this account, it has been proposed to improve the gas barrier properties by providing a hybrid gas barrier layer in which inorganic layers and polymer layers are laminated alternately (see Patent Documents 3 to 5, for example). However, because different processes are used to form the layers formed of the different materials, this method is not preferable in terms of production efficiency and cost. Furthermore, interlayer adhesion between the inorganic layers and the polymer layers is not high. Thus, there is a problem in that delamination may occur when the laminate is bent, resulting in deterioration in gas barrier properties. Accordingly, it is difficult to apply this method to flexible devices.